Oil, gas, water and geothermal wells are being drilled into the earth and normally such a bore hole is lined with steel and anchored by casting cement at the outside of these steel linings. Inside these steel walls equipment such as tubing, packers, side pocket mandrels, sliding side doors, surface or subsurface controlled valves and measurement tools can be installed either permanently or semi-permanently.
After the well has reached the end of its life either because of technical problems or because of becoming uneconomic or because of a license expiration, the well must be abandoned. There is typically a legal or contractual obligation to abandon the well in a specific manner and typically governmental guidelines or law describe precisely how a well must be abandoned. The typical procedure comprises retrieval of the tubing followed by removing the top of the well. This is the common practice for vertical wells, where the equipment is normally removed by simple retrieval.
Over many years the industry has developed methods to drill horizontal wells and has deployed this well type throughout the world. At the same time the industry has developed permanently and semi permanently installed equipment in this horizontal section, which cannot be retrieved as easily as the tubing of a vertical well. In some cases it is even physically impossible to retrieve these components due to obstructions in the well or partial collapse of the cemented lining, or because parts of the well have been corroded.
If the equipment cannot be removed by simple retrieval, e.g. because of the above problems, a downhole milling tool can be employed, which can mill the downhole equipment into small particles. U.S. Pat. No. 5,778,995 describes a downhole milling tool. Removing downhole equipment by milling requires the introduction of more advanced downhole equipment, as well as operation and maintenance of the milling tool. Furthermore if parts of the well have been partially obstructed by a collapse, the milling tool will not be able to function as intended.
U.S. Pat. No. 2,436,198 discloses a method which relate to chemical removal of an acid soluble metal part in a deep well. One object of the invention of U.S. Pat. No. 2,436,198 is to provide an improved method of, and composition for, chemically dissolving an aluminium or aluminium alloy part, such as a casing section, in the bore of a well whereby complete rapid removal is achieved. Dissolution of parts or equipment made of Al or Al-alloys in the well is achieved by subjecting the metal part of the corroding action of a hydrochloric acid solution to which has been added a relatively small amount of a phosphorus acid such as phosphoric acid (H3PO4) and hypo-phosphorous acid (H(H2PO2)). To prevent or reduce attack by the acid solution on adjacent ferrous metal parts, when such are present, an inhibitor of such action may be included in the acid solution.
U.S. Pat. No. 2,261,292 discloses a method for completing wells which traverse a plurality of producing horizons and has as particular object a completion procedure which will enable the operator to produce from various horizons simultaneously. According to the method comprise the string of casing which is set has one or more sections arranged so as to be opposite the upper producing horizons, and composed of a metal or a material which can be readily removed chemically. For example the material may be an aluminium alloy or a magnesium alloy or it may be an acid or alkali soluble resin. The chemical is an acid or a strong alkali e.g. hydrochloric acid.
U.S. Pat. No. 4,890,675 discloses a method for drilling of horizontal boreholes through formations traversed by a cased well. According to the method is provided a casing section adjacent to the formation which section is readily soluble in a selected chemical solution contacting the casing section with the selected chemical solution to dissolve the casing section and provide a “window” to the formation, and then drilling at least one generally horizontal borehole through the window into the formation. The removable section can be formed of Al or Mg, or an alloy of Al or Mg. The selected chemical solution may be an acid or an alkali. To minimize damage to the rest of the casing, a caustic solution is preferred. A strong hydroxide with alkali metal or ammonium nitrate is particularly effective in dissolving Al or Mg.
US 2005/0205266 relates to biodegradable downhole tools i.e. disposable tools, such as frac plugs and methods of removing such tools from wellbores. The disposable downhole tool or a component of the tool can comprise a degradable polymer e.g. an aliphatic polyester.
There exists a need for an improvement of the existing methods for the removal of downhole equipment that does not suffer the drawbacks described above.